1. Field of the Invention
The present invention concerns a image forming methods using a photothermographic material and, more specifically, it relates to a image forming methods at high line speed during exposure and thermal development, a image forming methods at rapid starting-up and a rapid image forming method with short developing time.
2. Description of the Related Art
In recent years, decrease for the amount of processing liquid wastes in the field of films for medical imagings and field of films for graphic arts has been keenly desired with a view point for environmental protection and space saving. Then, it has been required for techniques regarding photothermographic materials as films for medical imagings and films for graphic arts that can be exposed efficiently by laser image setters or laser imagers and can form clear black-toned images of high resolution and sharpness. According to the photothermographic materials described above, thermal development systems not requiring processing chemicals, simpler and not deteriorating environments can be supplied to customers.
While similar requirements exist also in the field of usual image forming materials, since fine expression is required particularly in images for medical imagings, high image quality of excellent sharpness and granularity are required, as well as images of blue-black tones are preferred with a view point of easy diagnosis. At present, various kinds of hard copy systems utilizing pigments and dyes such as ink jet printers and electrophotographs have been marketed as usual image forming systems at present, they are not satisfactory as output systems for medical images.
On the other hand, thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver systems” (Imaging Processes and Materials), Neblette, 8th edition, written by D. Klosterboer, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, page 279 in 1989.
Particularly, the photothermographic material generally comprises a photosensitive layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a silver salt capable of being reduced (for example, an organic silver salt) and, optionally, a toner for controlling the tone of developed silver image dispersed in a matrix of a binder.
The photothermographic material, when heated at high temperature (for example, 80° C. or higher) after imagewise exposure, forms black-toned silver images by oxidation/reduction reaction between a silver salt capable of being reduced (functioning as an oxidizer) and a reducing agent. The oxidation/reduction reaction is promoted by a catalytic activity of latent images of silver halide formed by exposure. Accordingly, black-toned silver images are formed in an exposed region. The photothermographic material has been described in U.S. Pat. No. 2,910,377 and JP-B No. 43-4924, as well as in many other literatures.
Also in the photothermographic material described above, it is usually required to improve the performance for thermal development processing and shorten the processing time.
Systems using laser beams such as laser imagers can continuously output photosensitive materials and are required for stability to the continuous output but stable output is difficult at present. While the photothermographic material is thermally developed at a high temperature of 100° C. or higher by being in contact with a 2-dimensional plane heater as a heat source in an automatic thermal developing apparatus, when most of photothermographic materials are thermal developed continuously, particularly, when various sizes of materials are processed continuously, since there exists delicate temperature difference between a portion of the plane heater in contact with the photosensitive material and a portion not in contact with the photosensitive material just before, this causes a problem of bringing about developer streaks to the photosensitive material to be developed subsequently. This is conspicuous, for example, in a case of processing large sized photosensitive materials immediately after processing small sized photosensitive materials. The developer streaks due to slight uneven heating result in difference in the tone of one sheet of photosensitive material to lower the stability of outputted images.
In the photothermographic material, it has been demanded that various sizes of materials can be developed in a great amount efficiently and the problem is significant.
On the other hand, for improving the processing performance and shortening the processing time, it has been demanded to increase the transportation speed (line speed) during development.
However, when the line speed is increased, temperature control for a thermally developing plate cooled by a cold photosensitive material can not be in-time and, since the state of development is different between the top end and the rear end of the heat treatment in one sheet of photosensitive material. This results in bringing about developer streaks for the photosensitive material. Such fine developer streaks causes difference in the tone in one single photosensitive material like the problem during continuous output to lower the stability of the output images to result in a significant problem.
Further, in a case where it is intended to develop just after the turning-on of a power source in a developing apparatus, temperature at the developing station of the developing apparatus is not stabilized to often cause a problem of developer streaks in the output images. It takes a considerable time from the starting of the developing apparatus by the turning-on of the power source till reaching the development temperature condition capable of obtaining stable images (referred to as times for starting-up in the present application), it is another subject in the rapid processing to shorten the times for starting-up.
Further, since all the chemicals required for development are incorporated in the photothermographic material, there is a problem caused by them, particularly, a problem of lowering performance and quality due to non-photochemical reactions between chemicals necessary for the development processing and photosensitive or image forming materials.
Among all, the problem concerning the image stability after the development processing of the photothermographic material includes a problem of worsening image storability caused by rapid processing (increase of fog with lapse of time after thermal development, that is, increase in the minimum density (Dmin) that greatly deteriorates the image quality. Since the thermal development time is short, ingredients such as organic silver salts and reducing agents in the photosensitive material can not be reacted completely to remain in the photosensitive material after development, and those ingredients react gradually during storage of processed images with lapse of time to increase Dmin.
Accordingly, it has been demanded to improve the rapid development processability without worsening the photographic performance and the image storability after processing.
As described above, it is an extremely difficult problem to compatibilize the rapid development processing and solution for various problems described above.